Spray the inside of a 6-quart slow cooker with cooking spray or line with parchment paper.
In a large bowl, combine peach slices, 1/2 cup sugar substitute, lemon juice, vanilla extract, cinnamon, nutmeg, and cornstarch. Toss until peaches are evenly coated, then pour into the slow cooker.
In a separate bowl, whisk together almond flour, coconut flour, 1/3 cup sugar substitute, baking powder, and salt.
Add melted butter, almond milk, and egg to the dry ingredients and stir until just combined.
Drop spoonfuls of the batter evenly over the peach mixture, spreading gently but not completely covering the fruit.
Cover and cook on LOW for 3 hours, or until the topping is set and the peaches are bubbling around the edges.
Turn off the slow cooker, remove the lid, and let stand for 15 minutes before serving to allow the dessert to set up properly.